Imaging lens

ABSTRACT

An imaging lens includes a first lens group; a second lens group; and a third lens group, arranged from an object side to an image plane side. The first lens group includes a positive first lens, a negative second lens, and a third lens. The second lens group includes a fourth lens and a fifth lens. The third lens group includes a sixth lens and a seventh lens. The first to seventh lenses are arranged on an optical axis with a space in between respectively. The seventh lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side have positive curvature radii. The surface of the seventh lens on the image plane side is formed in an aspheric shape having at least one inflexion point. The first and third lenses have specific Abbe&#39;s numbers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of a prior application Ser. No.15/275,557, filed on Sep. 26, 2016, allowed, which is a continuationapplication of a prior application Ser. No. 14/572,832, issued on Jan.10, 2017 as U.S. Pat. No. 9,541,730.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image ofan object on an imaging element such as a CCD sensor and a CMOS sensor.Particularly, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a camera to be builtin a portable device including a cellular phone and a portableinformation terminal, a digital still camera, a security camera, avehicle onboard camera, and a network camera.

In these years, in place of cellular phones that are intended mainly formaking phone calls, so-called smartphones have been more widely used,i.e., cellular phones with such functions as those of portableinformation terminals (PDA) and/or personal computers. Since thesmartphones generally are highly functional as opposed to the cellularphones, it is possible to use images taken by a camera thereof invarious applications.

Generally speaking, product groups of cellular phones and smartphonesoften include various designs from the ones for beginners to the onesfor advanced users. Among them, an imaging lens to be mounted in aproduct, which is developed for advanced users, is required to have ahigh resolution lens configuration so as to be also applicable to a highpixel count imaging element of these days.

In order to achieve a high resolution imaging lens, there is a method ofincreasing the number of lenses that compose the imaging lens. However,increase of the number of lenses easily causes increase in the size ofthe imaging lens, so that it is not preferred for the imaging lens to bemounted in a small-sized camera, such as the ones of cellular phones andsmartphones. For this reason, the imaging lens has been conventionallydeveloped in view of restraining the number of lenses as small aspossible.

In recent years, a technology for increasing a pixel count of an imagingelement has been dramatically advanced. A main focus in developing theimaging lens has been changing to achieving a lens configuration havinghigh resolution, rather than shortening a total track length. Forexample, there has been available a camera unit capable of obtaining aquality image even in comparison with that taken by a digital stillcamera by attaching the camera unit to a cellular phone, a smartphone,or the like, instead of mounting the camera unit that includes animaging lens and an imaging element in a cellular phone, a smartphone,or the like, which is conventionally done.

Although a seven-lens configuration could be slightly disadvantageous inview of downsizing the imaging lens due to the large number of lensesthat compose the imaging lens, there is high flexibility in designing.Therefore, the seven-lens configuration may have potential to achievesatisfactory correction of aberrations and downsizing of the imaginglens in a balanced manner. As the imaging lens having such a seven-lensconfiguration, for example, there is known one that is described inPatent Reference.

Patent Reference: Japanese Patent Application Publication No.2012-155223

According to Patent Reference, the conventional imaging lens includes afirst lens having a biconvex shape; a second lens that is joined to thefirst lends and has a biconcave shape; a third lens that is negative andhas a shape of a meniscus lens directing a convex surface thereof to theobject side; a fourth lens that is positive and has a shape of ameniscus lens directing a concave surface thereof to the object side; afifth lens that is negative and has a convex surface directing to theobject side; a sixth lens having a biconvex shape; and a seventh lenshaving a biconcave shape, arranged in the order from the object side.According to the conventional imaging lens disclosed in PatentReference, through restraining a ratio between a focal length of a firstlens group composed of the lenses from the first lens to the fourth lensand a focal length of a second lens group composed of the lenses fromthe fifth lens to the seventh lens within a certain range, it ispossible to attain downsizing of the imaging lens and satisfactorycorrection of aberrations.

According to the conventional imaging lens disclosed in PatentReference, although the size of the imaging lens is small, correction ofan image surface is not sufficient and especially distortion isrelatively large. Therefore, there is a limit by itself in achieving ahigh-resolution imaging lens. According to the lens configuration of theconventional imaging lens described in Patent Reference, it is difficultto more satisfactorily correct aberrations while downsizing the imaginglens.

Here, the above-described problem is not only specific to the imaginglens to be mounted in cellular phones and smart phones. Rather, it is acommon problem even for an imaging lens to be mounted in a relativelysmall camera such as digital still cameras, portable informationterminals, security cameras, vehicle onboard cameras, and networkcameras.

In view of the above-described problems in conventional techniques, anobject of the present invention is to provide an imaging lens that canattain both downsizing and satisfactory correction of aberrations.

Further objects and advantages of the invention will be apparent fromthe following description of the invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to a firstaspect of the present invention, an imaging lens includes a first lensgroup having positive refractive power; a second lens group havingpositive refractive power; and a third lens group having negativerefractive power, arranged in the order from an object side to an imageplane side. The first lens group includes a first lens having positiverefractive power; a second lens having negative refractive power; and athird lens having negative refractive power. The second lens groupincludes a fourth lens and a fifth lens. The third lens group includes asixth lens and a seven lens.

According to the first aspect of the invention, when the first lens hasAbbe's number νd1, the second lens has Abbe's number νd2, and the thirdlens has Abbe's number νd3, the imaging lens satisfies the followingconditional expressions (1) to (3)40<νd1<75  (1)20<νd2<35  (2)20<νd3<35  (3)

As described above, according to the first aspect of the invention, theimaging lens includes the first lens group having positive refractivepower, the second lens group having positive refractive power similarlyto the first lens group, and the third lens group having negativerefractive power, arranged in the order from the object side. Therefractive powers of the respective lens groups are arranged as“positive-positive-negative” from the object side. Typically, achromatic aberration is corrected by disposing a lens group havingpositive refractive power and a lens group having negative refractivepower in the order from the object side. In case of the lensconfiguration like this, in order to downsize the imaging lens, it isnecessary to increase the refractive power of the lens group that haspositive refractive power and is disposed on the object side. However,when the refractive power of the lens group having positive refractivepower is strong, it is often difficult to satisfactorily correct achromatic aberration.

According to the first aspect of the invention, in the imaging lens, thepositive refractive power of the whole lens system is divided betweenthe first lens group and the second lens group. Therefore, in comparisonwith when only one lens group has positive refractive power, it ispossible to relatively weakly retain the refractive power of thepositive lenses that compose the respective lens groups. Therefore,according to the imaging lens of the invention, among aberrations, it isespecially possible to satisfactorily correct the chromatic aberration.Moreover, it is achievable to obtain satisfactory image-formingperformance that is necessary for high-resolution imaging lens.Furthermore, according to the imaging lens of the invention, the thirdlens group has negative refractive power, so that it is possible tosuitably achieve downsizing of the imaging lens.

As described above, according to the first aspect of the invention, thefirst lens group includes three lenses, such that the arrangement ofrefractive powers thereof is positive-negative-negative. Those threelenses are made of lens materials that satisfy the conditionalexpressions (1) to (3). As a result, lens materials of the first lens,the second lens, and the third lens are a combination of alow-dispersion material and high-dispersion materials. With sucharrangement of the refractive powers and the order of Abbe's numbers ofthe respective lenses, it is achievable to suitably restrain generationof the chromatic aberration in the first lens group, and satisfactorilycorrect the chromatic aberration if generated. Here, according to theimaging lens of the invention, the negative refractive power is sharedbetween two lenses, i.e., the second lens and the third lens, so thatindividual refractive powers of the second lens and the third lens arerelatively weak. Since aberrations are corrected stepwise by the twolenses having relatively weak refractive powers, it is possible to moresatisfactorily correct the chromatic aberration than a lensconfiguration in which one negative lens is disposed on a side of animage surface of the first lens.

According to a second aspect of the invention, when the fourth lens haspositive refractive power, the fifth lens has negative refractive power,the fourth lens has Abbe's number νd4 and the fifth lens has Abbe'snumber νd5, the imaging lens having the above-described configurationpreferably satisfies the following conditional expressions (4) and (5):40<νd4<75  (4)20<νd5<35  (5)

According to the second aspect of the invention, the second lens groupis composed of two lenses that are a positive lens and a negative lens.In addition, the second lens group is composed of a combination of alens made of a low-dispersion material and a lens made of ahigh-dispersion material so as to satisfy the conditional expressions(4) and (5). As a result, among the aberrations generated in the firstlens group, especially the chromatic aberration is more satisfactorilycorrected. In general, in order to achieve a high-resolution imaginglens, among aberrations, it is necessary to satisfactorily correctespecially a chromatic aberration. According to the imaging lens of theinvention, the arrangement of the refractive powers of the respectivelens groups, the first to the third lens groups, and the arrangement ofthe refractive powers and the order of Abbe's numbers of the threelenses that compose the first lens group, and the arrangement of therefractive powers and the order of Abbe's numbers of the two lenses thatcompose the second lens group, it is achievable to more satisfactorilycorrect the chromatic aberration than a conventional imaging lens.

According to the imaging lens having the above-described configuration,the seventh lens is preferably formed, so as to have a negativerefractive power and so as to be formed in a shape such that thepositive refractive power increases as it goes to a lens periphery.

According to such shape of the seventh lens, it is achievable tosatisfactorily correct not only an axial chromatic aberration, but alsoan off-axis chromatic aberration of magnification. Moreover, as is wellknown, in case of an imaging element such as a CCD sensor and a CMOSsensor, there is set in advance a range of an incident angle of a lightbeam that can be taken in a sensor (so-called “chief ray angle (CRA)”).With a lens shape like that of the above-described seventh lens, it isachievable to suitably restrain the incident angle of a light beamemitted from the imaging lens to the image plane within the range ofCRA. As a result, it is achievable to suitably restrain generation ofshading, which is a phenomenon of becoming dark on the image periphery.

According to a third aspect of the invention, when the first lens has afocal length f1, and a composite focal length of the second lens and thethird lens is f23, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(6):−4.0<f23/f1<−1.5  (6)

When the imaging lens satisfies the conditional expression (6), it isachievable to restrain astigmatism, a field curvature, and a chromaticaberration within satisfactory ranges in a balanced manner, whiledownsizing the imaging lens. When the value exceeds the upper limit of“−1.5”, the negative refractive powers of the second lens and the thirdlens are relatively strong than the positive refractive power of thefirst lens. Therefore, it is advantageous for correction of thechromatic aberration. On the other hand, since the back focal length islong, it is difficult to downsize the imaging lens. Moreover, in theastigmatism, a sagittal image surface and a tangential image surfaceboth tilt to the side of the image plane (in a plus direction). As aresult, it is difficult to correct the astigmatism and the fieldcurvature increases, and therefore it is difficult to obtainsatisfactory image-forming performance. On the other hand, when thevalue is below the lower limit of “−4.0”, although it is advantageousfor downsizing of the imaging lens, the back focal length is short. As aresult, it is difficult to secure a space for disposing an insert suchas an infrared cutoff filter. In addition, the image-forming surfacecurves to the object side and the field curvature increases. Therefore,it is difficult to obtain satisfactory image-forming performance.

According to a fourth aspect of the invention, when the second lens hasa focal length f2, and a third lens has a focal length f3, the imaginglens having the above-described configuration preferably satisfies thefollowing conditional expression (7):0.1<f2/f3<0.6  (7)

When the imaging lens satisfies the conditional expression (7), it isachievable to satisfactorily correct the astigmatism and the fieldcurvature. When the value exceeds the upper limit of “0.6”, in the firstlens group, the negative refractive power of the second lens, which isclose to the first lens that is positive, is relatively weak. Therefore,in the astigmatism, the tangential image surface curves to the objectside (in a minus direction), and an astigmatic difference increases. Asa result, it is difficult to correct the astigmatism. Moreover, sincethe image-forming surface curves in a minus direction, it is difficultto obtain satisfactory image-forming performance. On the other hand,when the value is below the lower limit of “0.1”, in the astigmatism,the tangential image surface curves in a plus direction, and theastigmatic difference increases. As a result, it is difficult to correctthe astigmatism. In addition, the image-forming surface curves in theplus direction, also in this case, it is difficult to obtainsatisfactory image-forming performance.

According to a fifth aspect of the invention, when the whole lens systemhas a focal length f and the first lens has a focal length f1, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (8):0.5<f1/f<2.0  (8)

When the imaging lens satisfies the conditional expression (8), it isachievable to satisfactorily correct the astigmatism, the fieldcurvature, and the distortion, while downsizing the imaging lens. Inaddition, when the imaging lens satisfies the conditional expression(8), it is also achievable to restrain the incident angle of a lightbeam emitted from the imaging lens to the image plane within the rangeof CRA. When the value exceeds the upper value of “2.0”, the positiverefractive power of the first lens is weak relative to the refractivepower of the whole lens system. As a result, the positive refractivepower of the second lens group is relatively strong. For this reason, itis easy to restrain the incident angle of a light beam emitted from theimaging lens to the image plane within the range of CRA. But in theastigmatism, the sagittal image surface curves in a minus direction andthe field curvature is insufficiently corrected (an image-formingsurface curves in a minus direction). Therefore, it is difficult toobtain satisfactory image-forming performance. On the other hand, whenthe value is below the lower limit of “0.5”, although it is advantageousfor downsizing of the imaging lens, it is difficult to secure the backfocal length. In addition, it is difficult to restrain the incidentangle of a light beam emitted from the imaging lens to the image planewithin the range of CRA. Furthermore, in this case, the plus distortionincreases, so that it is difficult to obtain satisfactory image-formingperformance.

According to a sixth aspect of the invention, when a composite focallength of the fourth lens and the fifth lens is f45 and a compositefocal length of the sixth lens and the seventh lens is f67, the imaginglens having the above-described configuration preferably satisfies thefollowing conditional expression (9):−6.0<f45/f67<−1.5  (9)

When the imaging lens satisfies the conditional expression (9), it isachievable to restrain the distortion, the chromatic aberration, and thefield curvature within their respective satisfactory ranges, whiledownsizing the imaging lens. When the value exceeds the upper limit of“−1.5”, the negative refractive power of the third lens group is weakrelative to the positive refractive power of the second lens group.Therefore, it is necessary to weaken the positive refractive power ofthe first lens group. When the refractive power of the first lens groupis weak, it is easy to secure the back focal length, but it is difficultto downsize the imaging lens. Moreover, the minus distortion and thechromatic aberration of magnification increase, and the filed curvatureis insufficiently corrected. Therefore, it is difficult to obtainsatisfactory image-forming performance. On the other hand, when thevalue is below the lower limit of “−6.0”, although it is advantageousfor correction of the chromatic aberration and correction of the fieldcurvature, a plus distortion increases. Therefore, it is difficult toobtain satisfactory image-forming performance.

According to a seventh aspect of the invention, when the whole lenssystem has a focal length f, and a composite focal length of the sixthlens and the seventh lens is f67, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (10):−1.7<f67/f<−0.5  (10)

When the imaging lens satisfies the conditional expression (10), it isachievable to satisfactorily correct the distortion, the chromaticaberration, and the astigmatism in a balanced manner, while restrainingthe incident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA. When the value exceeds the upperlimit of “−0.5”, the refractive power of the third lens group is strongrelative to that of the whole lens system. For this reason, thechromatic aberration of magnification increases at image periphery andthe plus distortion increases. As a result, it is difficult to obtainsatisfactory image-forming performance. In addition, it is difficult torestrain the incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. On the other hand, whenthe value is below the lower limit of “−1.7”, although it isadvantageous for correction of the distortion, in the astigmatism, thesagittal image surface curves to the object side and the chromaticaberration of magnification increases. Therefore, it is difficult toobtain satisfactory image-forming performance.

According to an eighth aspect of the invention, when the sixth lens hasAbbe's number νd6 and the seventh lens has Abbe's number νd7, theimaging lens having the above-described configuration preferablysatisfies the following conditional expressions (11) and (12):40<νd6<75  (11)40<νd7<75  (12)

The sixth lens and the seventh lens compose the third lens group, whichis the lens group that is closest to the image plane side. When thesixth lens and the seventh lens are made of low-dispersion materialsthat satisfy the conditional expressions (11) and (12), it is achievableto suitably restrain the chromatic dispersion at the respective lensesand satisfactorily correct the aberrations.

According to a ninth aspect of the invention, when the fourth lens haspositive refractive power, the whole lens system has a focal length f,and the fourth lens has a focal length f4, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (13):1.0<f4/f<3.0  (13)

When the imaging lens satisfies the conditional expression (13), it isachievable to satisfactorily correct an off-axis coma aberration and thedistortion, while downsizing the imaging lens. In addition, when theimaging lens satisfies the conditional expression (13), it is alsopossible to restrain the incident angle of a light beam emitted from theimaging lens to the image plane within the range of CRA. When the valueexceeds the upper limit of “3.0”, the positive refractive power of thefourth lens is weak relative to that of the refractive power of thewhole lens system. Therefore, in order to satisfactorily correct theaberrations, it is necessary to relatively increase the positiverefractive power of the first lens group. In this case, although it isadvantageous for downsizing of the imaging lens, it is difficult tosecure the back focal length. Furthermore, the plus distortion increasesand an inner coma aberration easily occurs on a tangential surface of anoff-axis beam of light. As a result, it is difficult to obtainsatisfactory image-forming performance. On the other hand, when thevalue is below the lower limit of “1.0”, although it is easy to restrainthe incident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA, it is difficult to downsize theimaging lens. In addition, a coma aberration easily occurs on a sagittalsurface and on a tangential surface of an off-axis beam of light.Therefore, it is difficult to obtain satisfactory image-formingperformance.

According to a tenth aspect of the invention, when the seventh lens hasnegative refractive power, the whole lens system has a focal length f,and the seventh lens has a focal length f7, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (14):−4.0<f7/f<−0.8  (14)

When the imaging lens satisfies the conditional expression (14), it isachievable to satisfactorily correct the chromatic aberration and thefield curvature, while downsizing the imaging lens. In addition, whenthe imaging lens satisfies the conditional expression (14), it is alsoachievable to restrain the incident angle of a light beam emitted fromthe imaging lens to the image plane within the range of CRA. When thevalue exceeds the upper limit of “−0.8”, although it is advantageous forcorrection of the chromatic aberration and downsizing of the imaginglens, the field curvature is excessively corrected (an image-formingsurface curves in a plus direction). As a result, it is difficult toobtain satisfactory image-forming performance. In addition, it is alsodifficult to restrain the incident angle of a light beam emitted fromthe imaging lens to the image plane within the range of CRA. On theother hand, when the value is below the lower limit of “−4.0”, althoughit is easy to restrain the incident angle of a light beam emitted fromthe imaging lens to the image plane within the range of CRA, it isdifficult to downsize the imaging lens. In addition, the chromaticaberration of magnification increases at image periphery, and the fieldcurvature is insufficiently corrected. As a result, it is difficult toobtain satisfactory image-forming performance.

According to an eleventh aspect of the invention, when the whole lenssystem has a focal length f, a distance on an optical axis between thesecond lens and the third lens is D23, the imaging lens having theabove-describe configuration preferably satisfies the followingconditional expression (15):0.03<D23/f<0.2  (15)

When the imaging lens satisfies the conditional expression (15), it isachievable to satisfactorily correct the field curvature, thedistortion, and the chromatic aberration, while downsizing the imaginglens. When the value exceeds the upper limit of “0.2”, although it isadvantageous for downsizing of the imaging lens, it is difficult tosecure the back focal length. In addition, the plus distortionincreases, and the field curvature is excessively corrected. Moreover,the chromatic aberration of magnification increases at image periphery,and it is difficult to obtain satisfactory image-forming performance. Onthe other hand, when the value is below the lower limit of “0.03”,although it is easy to secure the back focal length, the field curvatureis insufficiently corrected, and the chromatic aberration ofmagnification increases. As a result, it is difficult to obtainsatisfactory image-forming performance.

According to a twelfth aspect of the invention, when the whole lenssystem has a focal length f and a distance on the optical axis betweenthe third lens and the fourth lens is D34, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (16):0.03<D34/f<0.2  (16)

When the imaging lens satisfies the conditional expression (16), it isachievable to restrain the field curvature, the distortion, and thechromatic aberration of magnification within satisfactory ranges in abalanced manner. When the value exceeds the upper limit of “0.2”,although it is easy to restrain the chromatic aberration ofmagnification generated at image periphery, the field curvature isinsufficiently corrected and the plus distortion increases. As a result,it is difficult to obtain satisfactory image-forming performance. On theother hand, when the value is below the lower limit of “0.03”, althoughit is easy to secure the back focal length, the field curvature isexcessively corrected. As a result, it is difficult to obtainsatisfactory image-forming performance.

According to the imaging lens of the invention, it is possible toprovide a small-sized imaging lens, which is especially suitable formounting in a small camera, while having high resolution withsatisfactorily corrected aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 1 according to an embodiment of theinvention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 2 according to the embodiment of theinvention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 3 according to the embodiment of theinvention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 4 according to the embodiment ofthe invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 5 according to the embodiment ofthe invention;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 13; and

FIG. 15 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 13;

FIG. 16 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 6 according to the embodiment ofthe invention;

FIG. 17 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 16; and

FIG. 18 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 16.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, an embodiment of thepresent invention will be fully described.

FIGS. 1, 4, 7, 10, 13, and 16 are schematic sectional views of theimaging lenses in Numerical Data Examples 1 to 6 according to theembodiment, respectively. Since a basic lens configuration is the sameamong those Numerical Data Examples, the lens configuration of theembodiment will be described with reference to the illustrativesectional view of Numerical Data Example 1.

As shown in FIG. 1, the imaging lens of the embodiment includes a firstlens group G1 having positive refractive power; a second lens group G2having positive refractive power; and a third lens group G3 havingnegative refractive power, arranged in the order from an object side toan image plane side. Between the third lens group G3 and an image planeIM, there is disposed a filter 10. The filter 10 may be optionallyomitted.

The first lens group G1 includes a first lens L1 having positiverefractive power; a second lens L2 having negative refractive power; anda third lens L3 having negative refractive power, arranged in the orderfrom the object side. The first lens L1 is formed in a shape such that acurvature radius r1 of an object-side surface thereof and a curvatureradius r2 of an image plane-side surface thereof are both positive andso as to have a shape of a meniscus lens directing a convex surfacethereof to the object side near an optical axis X. The shape of thefirst lens L1 is not limited to the one in Numerical Data Example 1. Asan example of another shape of the first lens L1, Numerical Data Example6 shows the one, which is formed in a shape such that the curvatureradius r1 and the curvature radius r2 are both negative, i.e., a shapeof a meniscus lens directing a concave surface thereof to the objectside near the optical axis X. As another example, the first lens L1 maybe formed in a shape such that the curvature radius r2 is negative,i.e., a shape of a biconvex lens near the optical axis X. Here, in orderto more effectively downsize the imaging lens, the first lens L1 ispreferably formed in a shape of a meniscus lens directing a convexsurface thereof to the object side near the optical axis X.

The second lens L2 is formed in a shape such that a curvature radius r3of an object-side surface is negative and a curvature radius r4 of animage plane-side surface is positive, and so as to have a shape of abiconcave lens near the optical axis X. Here, the shape of the secondlens L2 is not limited to the one in Numerical Data Example 1. The shapeof the second lens L2 may be any as long as the curvature radius r4 ofthe image plane-side surface thereof is positive, and can be formed in ashape such that the curvature radius r3 is positive, i.e., a shape of ameniscus lens directing a convex surface thereof to the object-side nearthe optical axis X. Numerical Data Examples 2 to 6 are examples, inwhich the shape of the second lens L2 is a meniscus lens directing aconvex surface thereof to the object side near the optical axis X.

The third lens L3 is formed in a shape, such that a curvature radius r5of the object-side surface thereof is negative and a curvature radius r6of an image plane-side surface thereof is positive, and so as to have ashape of a biconcave lens near the optical axis X. The shape of thethird lens L3 is also not limited to the one in Numerical DataExample 1. Numerical Data Example 3 is an example, in which the thirdlens L3 is formed in a shape such that the curvature radius r5 and thecurvature radius r6 are both negative and has a shape of a meniscus lensdirecting a concave surface thereof to the object side near the opticalaxis X. Numerical Data Example 6 is an example, in which the third lensL3 is formed in a shape such that the curvature radius r5 and thecurvature radius r6 are both positive and so as to have a shape of ameniscus lens directing a convex surface thereof to the object side nearthe optical axis X.

The second lens group G2 includes a fourth lens L4 having positiverefractive power and a fifth lens L5 having negative refractive power,arranged in the order from the object side. Between them, the fourthlens L4 is formed in a shape such that a curvature radius r7 of anobject-side surface thereof and a curvature radius r8 of an imageplane-side surface thereof are both negative, and so as to have a shapeof a meniscus lens directing a concave surface thereof to the objectside near the optical axis X. The fourth lens L4 can be formed in anyshapes as long as the refractive power is positive. As a shape of thefourth lens L4, for example, the fourth lens L4 can be formed in a shapesuch that the curvature radius r7 and the curvature radius r8 are bothpositive and so as to have a shape of a meniscus lens directing a convexsurface thereof to the object side near the optical axis X. NumericalData Example 6 is an example, in which the fourth lens L4 is formed in ashape of a meniscus lens directing a convex surface thereof to theobject side near the optical axis X.

On the other hand, the fifth lens L5 is formed in a shape such that acurvature radius r9 of the object-side surface thereof and a curvatureradius r10 of an image plane-side surface thereof are both positive andso as to have a shape of a meniscus lens directing a convex surfacethereof to the object side near the optical axis X. The fifth lens L5can have positive refractive power. Here, in view of satisfactorycorrection of aberrations, it is preferred that the refractive powers ofthe fourth lens L4 and the fifth lens L5 are not both positive/negative.

The third lens group G3 includes a sixth lens L6 having negativerefractive power and a seventh lens L7 having negative refractive power,arranged in the order from the object side. The sixth lens L6 is formedin a shape such that a curvature radius r11 of an object-side surfacethereof and a curvature radius r12 of an image plane-side surfacethereof are both positive, and so as to have a shape of a meniscus lensdirecting a convex surface thereof to the object side near the opticalaxis X. The seventh lens L7 is formed in a shape such that a curvatureradius r13 of an object-side surface thereof and a curvature radius r14of an image plane-side surface thereof are both positive, and so as tohave a shape of a meniscus lens directing a convex surface thereof tothe object side near the optical axis X. Here, the refractive power ofthe sixth lens L6 can be positive. Numerical Data Example 2 is anexample, in which the refractive power of the sixth lens L6 is positive.

In the sixth lens L6 and the seventh lens L7, the object-side surfacesthereof and the image-plane-side surfaces thereof are formed as asphericsurfaces having inflexion points, and so as to have strong positiverefractive powers as it goes to the lens periphery. With those shapes ofthe sixth lens L6 and the seventh lens L7, it is achievable tosatisfactorily correct not only an axial chromatic aberration, but alsoan off-axis chromatic aberration of magnification. In addition, it isalso achievable to suitably restrain an incident angle of a light beamemitted from the imaging lens to the image plane IM within the range ofa chief ray angle (CRA).

Here, according to the imaging lens of Numerical Data Example 1, thesixth lens L6 and the seventh lens L7 have their both object-sidesurfaces and image plane-side surfaces formed as aspheric shapes havinginflexion points, but it is not necessary to form those both surfaces asaspheric surfaces having inflexion points. Even when only one of thesurfaces is formed as an aspheric surface having an inflexion point, itis still possible to form both or one of the lenses in a shape so as tohave strong positive refractive power as it goes to the lens periphery.In addition, depending on the levels of required optical performance anddownsizing, it is not always necessary to provide an inflexion point onthe sixth lens L6 and the seventh lens L7.

The imaging lens of the embodiment satisfies the following conditionalexpressions (1) to (16):40<νd1<75  (1)20<νd2<35  (2)20<νd3<35  (3)40<νd4<75  (4)20<νd5<35  (5)−4.0<f23/f1<−1.5  (6)0.1<f2/f3<0.6  (7)0.5<f1/f<2.0  (8)−6.0<f45/f67<−1.5  (9)−1.7<f67/f<−0.5  (10)40<νd6<75  (11)40<νd7<75  (12)1.0<f4/f<3.0  (13)−4.0<f7/f<−0.8  (14)0.03<D23/f<0.2  (15)0.03<D34/f<0.2  (16)In the above conditional expressions:νd1: Abbe's number of a first lens L1νd2: Abbe's number of a second lens L2νd3: Abbe's number of a third lens L3νd4: Abbe's number of a fourth lens L4νd5: Abbe's number of a fifth lens L5νd6: Abbe's number of a sixth lens L6νd7: Abbe's number of a seventh lens L7f: Focal length of a whole lens systemf1: Focal length of the first lens L1f2: Focal length of the second lens L2f3: Focal length of the third lens L3f4: Focal length of the fourth lens L4f7: Focal length of the seventh lens L5f23: Composite focal length of the second lens L2 and the third lens L3f45: Composite focal length of the fourth lens L4 and the fifth lens L5f67: Composite focal length of the sixth lens L6 and the seventh lens L7D23 : Distance on the optical axis between the second lens L2 and thethird lens L3D34: Distance on the optical axis between the third lens L3 and thefourth lens L4

Here, it is not necessary to satisfy all of the conditional expressions,and it is achievable to obtain an effect corresponding to the respectiveconditional expression when any single one of the conditionalexpressions is individually satisfied.

In the embodiment, all lens surfaces of the respective lenses are formedas an aspheric surface. When the aspheric surfaces applied to the lenssurfaces have an axis Z in a direction of the optical axis X, a height Hin a direction perpendicular to the optical axis, a conical coefficientk, and aspheric coefficients A₄, A₆, A₈, A₁₀, A₁₂, A₁₄, and A₁₆, a shapeof the aspheric surfaces of the lens surfaces is expressed as follows:

$\begin{matrix}{Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}} + {A_{14}H^{14}} + {A_{16}H^{16}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each Numerical Data Example, f represents a focallength of the whole lens system, Fno represents an F number, and ωrepresents a half angle of view, respectively. In addition, i representsa surface number counted from the object side, r represents a curvatureradius, d represents a distance between lens surfaces (surface spacing)on the optical axis, nd represents a refractive index, and νd representsAbbe's number, respectively. Here, aspheric surfaces are indicated withsurface numbers i affixed with * (asterisk).

NUMERICAL DATA EXAMPLE 1

Basic data are shown below.

f = 7.18 mm, Fno = 3.0, ω = 29.1° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* (Stop) 2.137 0.876 1.5346 56.1 (=νd1)  2*12.503 0.082  3* −525.874 0.315 1.6355 24.0 (=νd2)  4* 8.206 0.602(=D23)  5* −116.694 0.856 1.6355 24.0 (=νd3)  6* 19.254 0.464 (=D34)  7*−4.712 0.762 1.5346 56.1 (=νd4)  8* −2.361 0.105  9* 17.655 0.510 1.635524.0 (=νd5) 10* 13.764 0.279 11* 12.387 0.483 1.5346 56.1 (=νd6) 12*6.010 0.418 13* 33.769 0.500 1.5346 56.1 (=νd7) 14* 3.536 0.200 15 ∞0.200 1.5168 64.2 16 ∞ 1.249 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = −1.269E−03, A₆ = −1.041E−03, A₈ = −6.082E−04,A₁₀ = −1.066E−03, A₁₂ = 5.675E−04, A₁₄ = −3.136E−04, A₁₆ = −1.053E−04Second Surface k = 0.000, A₄ = −7.459E−02, A₆ = 3.177E−02, A₈ =−1.722E−04, A₁₀ = −2.787E−03, A₁₂ = −1.500E−03, A₁₄ = −2.828E−04, A₁₆ =2.980E−04 Third Surface k = 0.000, A₄ = −7.996E−02, A₆ = 6.597E−02, A₈ =−5.343E−03, A₁₀ = −3.986E−03, A₁₂ = −6.224E−04, A₁₄ = 5.170E−04, A₁₆ =8.386E−05 Fourth Surface k = 0.000, A₄ = −2.982E−02, A₆ = 4.276E−02, A₈= −2.541E−03, A₁₀ = −2.635E−03, A₁₂ = 1.060E−03, A₁₄ = 2.993E−04, A₁₆ =4.110E−04 Fifth Surface k = 0.000, A₄ = −5.158E−02, A₆ = 2.715E−03, A₈ =−1.639E−03, A₁₀ = 1.341E−03, A₁₂ = 8.514E−04, A₁₄ = 5.026E−05, A₁₆ =−3.478E−05 Sixth Surface k = 0.000, A₄ = −1.784E−02, A₆ = 1.970E−03, A₈= 2.539E−04, A₁₀ = −3.310E−04, A₁₂ = −1.014E−04, A₁₄ = 2.088E−04, A₁₆ =−4.369E−05 Seventh Surface k = 0.000, A₄ = 2.912E−02, A₆ = −7.102E−03,A₈ = 5.929E−03, A₁₀ = −2.531E−04, A₁₂ = −3.260E−03, A₁₄ = 1.780E−03, A₁₆= −3.061E−04 Eighth Surface k = 0.000, A₄ = 8.806E−03, A₆ = 9.910E−03,A₈ = −4.654E−04, A₁₀ = −3.451E−03, A₁₂ = 2.664E−03, A₁₄ = −6.962E−04,A₁₆ = 5.407E−05 Ninth Surface k = 0.000, A₄ = −3.991E−02, A₆ =8.162E−03, A₈ = −5.350E−03, A₁₀ = −9.445E−05, A₁₂ = 9.071E−04, A₁₄ =−2.112E−04, A₁₆ = 1.218E−05 Tenth Surface k = 0.000, A₄ = −3.411E−02, A₆= −1.380E−03, A₈ = −6.512E−05, A₁₀ = 1.677E−05, A₁₂ = 1.676E−05, A₁₄ =2.441E−06, A₁₆ = −6.710E−07 Eleventh Surface k = 0.000, A₄ = −3.657E−02,A₆ = 9.329E−04, A₈ = 1.629E−04, A₁₀ = 1.382E−05, A₁₂ = 2.057E−06, A₁₄ =3.601E−07, A₁₆ = −8.717E−08 Twelfth Surface k = 0.000, A₄ = −1.325E−02,A₆ = −1.573E−04, A₈ = −8.403E−05, A₁₀ = −9.963E−07, A₁₂ = −1.366E−06,A₁₄ = −1.165E−07, A₁₆ = 3.426E−08 Thirteenth Surface k = 0.000, A₄ =−2.772E−02, A₆ = 7.343E−03, A₈ = −1.330E−03, A₁₀ = 1.028E−04, A₁₂ =−1.210E−05, A₁₄ = 1.664E−06, A₁₆ = −7.563E−08 Fourteenth Surface k =0.000, A₄ = −5.223E−02, A₆ = 9.961E−03, A₈ = −1.759E−03, A₁₀ =2.358E−04, A₁₂ = −2.025E−05, A₁₄ = 9.640E−07, A₁₆ = −2.007E−08 f1 = 4.68mm f2 = −12.71 mm f3 = −25.94 mm f4 = 7.95 mm f5 = −103.56 mm f6 =−22.43 mm f7 = −7.43 mm f23 = −8.31 mm f45 = 8.51 mm f67 = −5.49 mm Thevalues of the respective conditional expressions are as follows: f23/f1= −1.77 f2/f3 = 0.49 f1/f = 0.65 f45/f67 = −1.55 f67/f = −0.77 f4/f =1.11 f7/f = −1.04 D23/f = 0.084 D34/f = 0.065

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theabove-described conditional expressions. A distance on the optical axisfrom the object-side surface of the first lens L1 to the image plane IM(air conversion length for the filter 10) is 7.83 mm, and downsizing ofthe imaging lens is attained.

FIG. 2 shows a lateral aberration of the imaging lens in Numerical DataExample 1, which corresponds to a ratio H of each image height to themaximum image height (hereinafter referred to as “image height ratioH”), being divided into a tangential direction and a sagittal direction(which is the same in FIGS. 5, 8, 11, 14, and 17). Furthermore, FIG. 3shows a spherical aberration (mm), astigmatism (mm), and a distortion(%), respectively. In the aberration diagrams, for the lateralaberration diagrams and spherical aberration diagrams, aberrations ateach wavelength, i.e. a g line (436 nm), a d line (588 nm), and a C line(656 nm) are indicated. In astigmatism diagram, an aberration on asagittal image surface S and an aberration on a tangential image surfaceT are respectively indicated (which are the same in FIGS. 6, 9, 12, 15,and 18). As shown in FIGS. 2 and 3, according to the imaging lens ofNumerical Data Example 1, the aberrations are satisfactorily corrected.

NUMERICAL DATA EXAMPLE 2

Basic data are shown below.

f = 5.44 mm, Fno = 2.2, ω = 36.3° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* (Stop) 2.037 0.916 1.5346 56.1 (=νd1)  2*17.722 0.056  3* 89.558 0.297 1.6355 24.0 (=νd2)  4* 9.329 0.397 (=D23) 5* −43.041 0.579 1.6355 24.0 (=νd3)  6* 252.636 0.291 (=D34)  7* −4.0820.780 1.5346 56.1 (=νd4)  8* −2.345 0.037  9* 30.893 0.510 1.6355 24.0(=νd5) 10* 9.990 0.208 11* 7.396 0.561 1.5346 56.1 (=νd6) 12* 7.5290.236 13* 35.369 0.500 1.5346 56.1 (=νd7) 14* 3.585 0.200 15 ∞ 0.2001.5168 64.2 16 ∞ 0.832 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = 1.278E−03, A₆ = −1.344E−03, A₈ = −6.049E−04, A₁₀= −9.928E−04, A₁₂ = 5.549E−04, A₁₄ = −3.389E−04, A₁₆ = −1.053E−04 SecondSurface k = 0.000, A₄ = −7.369E−02, A₆ = 3.182E−02, A₈ = −2.362E−04, A₁₀= −2.772E−03, A₁₂ = −1.452E−03, A₁₄ = −2.374E−04, A₁₆ = 3.103E−04 ThirdSurface k = 0.000, A₄ = −8.231E−02, A₆ = 6.611E−02, A₈ = −5.107E−03, A₁₀= −3.861E−03, A₁₂ = −5.837E−04, A₁₄ = 5.444E−04, A₁₆ = 1.194E−04 FourthSurface k = 0.000, A₄ = −2.913E−02, A₆ = 4.127E−02, A₈ = −2.459E−03, A₁₀= −2.343E−03, A₁₂ = 1.134E−03, A₁₄ = 2.487E−04, A₁₆ = 3.403E−04 FifthSurface k = 0.000, A₄ = −5.270E−02, A₆ = 3.670E−03, A₈ = −1.855E−03, A₁₀= 9.253E−04, A₁₂ = 6.531E−04, A₁₄ = 5.241E−05, A₁₆ = 3.456E−05 SixthSurface k = 0.000, A₄ = −1.418E−02, A₆ = 2.764E−03, A₈ = 6.929E−04, A₁₀= −1.418E−04, A₁₂ = −2.387E−05, A₁₄ = 2.459E−04, A₁₆ = −2.662E−05Seventh Surface k = 0.000, A₄ = 3.184E−02, A₆ = −6.312E−03, A₈ =5.993E−03, A₁₀ = −2.065E−04, A₁₂ = −3.225E−03, A₁₄ = 1.791E−03, A₁₆ =−3.069E−04 Eighth Surface k = 0.000, A₄ = 9.352E−03, A₆ = 9.939E−03, A₈= −3.373E−04, A₁₀ = −3.412E−03, A₁₂ = 2.668E−03, A₁₄ = −6.963E−04, A₁₆ =5.389E−05 Ninth Surface k = 0.000, A₄ = −4.199E−02, A₆ = 7.392E−03, A₈ =−5.501E−03, A₁₀ = −1.110E−04, A₁₂ = 9.072E−04, A₁₄ = −2.104E−04, A₁₆ =1.245E−05 Tenth Surface k = 0.000, A₄ = −3.196E−02, A₆ = −8.737E−04, A₈= 3.452E−05, A₁₀ = 2.494E−05, A₁₂ = 1.701E−05, A₁₄ = 2.379E−06, A₁₆ =−6.898E−07 Eleventh Surface k = 0.000, A₄ = −3.494E−02, A₆ = 9.838E−04,A₈ = 1.285E−04, A₁₀ = 1.041E−05, A₁₂ = 1.781E−06, A₁₄ = 3.140E−07, A₁₆ =−1.017E−07 Twelfth Surface k = 0.000, A₄ = −1.149E−02, A₆ = 1.483E−04,A₈ = −8.259E−05, A₁₀ = −4.430E−06, A₁₂ = −1.733E−06, A₁₄ = −1.292E−07,A₁₆ = 3.717E−08 Thirteenth Surface k = 0.000, A₄ = −2.715E−02, A₆ =8.014E−03, A₈ = −1.326E−03, A₁₀ = 1.028E−04, A₁₂ = −1.206E−05, A₁₄ =1.663E−06, A₁₆ = −7.748E−08 Fourteenth Surface k = 0.000, A₄ =−4.908E−02, A₆ = 9.595E−03, A₈ = −1.765E−03, A₁₀ = 2.358E−04, A₁₂ =−2.026E−05, A₁₄ = 9.640E−07, A₁₆ = −1.996E−08 f1 = 4.22 mm f2 = −16.41mm f3 = −57.82 mm f4 = 8.91 mm f5 = −23.45 mm f6 = 317.09 mm f7 = −7.50mm f23 = −12.71 mm f45 = 14.46 mm f67 = −7.92 mm The values of therespective conditional expressions are as follows: f23/f1 = −3.01 f2/f3= 0.28 f1/f = 0.78 f45/f67 = −1.83 f67/f = −1.46 f4/f = 1.64 f7/f =−1.38 D23/f = 0.073 D34/f = 0.053

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theabove-described conditional expressions. A distance on the optical axisfrom the object-side surface of the first lens L1 to the image plane IM(air conversion length for the filter 10) is 6.53 mm, and downsizing ofthe imaging lens is attained.

FIG. 5 shows the lateral aberration of the imaging lens in NumericalData Example 2, which corresponds to the image height ratio H of theimaging lens. FIG. 6 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively. As shown in FIGS. 5 and 6,according to the imaging lens of Numerical Data Example 2, theaberrations are also satisfactorily corrected.

NUMERICAL DATA EXAMPLE 3

Basic data are shown below.

f = 6.32 mm, Fno = 2.6, ω = 32.3° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* (Stop) 2.081 0.680 1.5346 56.1 (=νd1)  2*20.921 0.080  3* 70.779 0.290 1.6355 24.0 (=νd2)  4* 8.233 1.201 (=D23) 5* −7.147 0.343 1.6355 24.0 (=νd3)  6* −10.623 0.221 (=D34)  7* −6.2480.414 1.5346 56.1 (=νd4)  8* −3.507 0.047  9* 17.703 0.510 1.6355 24.0(=νd5) 10* 8.206 0.283 11* 11.066 0.677 1.5346 56.1 (=νd6) 12* 8.9790.247 13* 33.195 0.963 1.5346 56.1 (=νd7) 14* 3.725 0.200 15 ∞ 0.2001.5168 64.2 16 ∞ 0.628 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = 2.316E−03, A₆ = −6.966E−04, A₈ = −8.666E−04, A₁₀= −8.910E−04, A₁₂ = 7.588E−04, A₁₄ = −3.174E−04, A₁₆ = −3.121E−04 SecondSurface k = 0.000, A₄ = −6.708E−02, A₆ = 3.193E−02, A₈ = 1.178E−04, A₁₀= −3.199E−03, A₁₂ = −1.616E−03, A₁₄ = −1.742E−04, A₁₆ = 3.390E−04 ThirdSurface k = 0.000, A₄ = −7.504E−02, A₆ = 6.484E−02, A₈ = −7.363E−03, A₁₀= −4.744E−03, A₁₂ = −6.707E−04, A₁₄ = 6.906E−04, A₁₆ = 3.379E−04 FourthSurface k = 0.000, A₄ = −5.473E−03, A₆ = 4.021E−02, A₈ = −4.702E−03, A₁₀= −2.717E−03, A₁₂ = 1.284E−03, A₁₄ = 3.257E−04, A₁₆ = 7.159E−05 FifthSurface k = 0.000, A₄ = −3.325E−02, A₆ = 7.261E−03, A₈ = 4.647E−04, A₁₀= 1.109E−03, A₁₂ = 2.867E−04, A₁₄ = −1.887E−05, A₁₆ = −2.570E−05 SixthSurface k = 0.000, A₄ = −4.105E−02, A₆ = 3.279E−05, A₈ = 1.900E−03, A₁₀= 2.365E−04, A₁₂ = 3.118E−05, A₁₄ = 2.388E−04, A₁₆ = −2.065E−05 SeventhSurface k = 0.000, A₄ = −9.376E−03, A₆ = −1.369E−02, A₈ = 7.969E−03, A₁₀= −4.465E−06, A₁₂ = −3.255E−03, A₁₄ = 1.808E−03, A₁₆ = −2.768E−04 EighthSurface k = 0.000, A₄ = −6.996E−03, A₆ = 1.005E−02, A₈ = −1.165E−03, A₁₀= −3.518E−03, A₁₂ = 2.675E−03, A₁₄ = −6.821E−04, A₁₆ = 5.876E−05 NinthSurface k = 0.000, A₄ = −2.105E−02, A₆ = 2.209E−03, A₈ = −4.117E−03, A₁₀= −1.343E−04, A₁₂ = 8.072E−04, A₁₄ = −2.255E−04, A₁₆ = 1.704E−05 TenthSurface k = 0.000, A₄ = −1.872E−02, A₆ = −1.529E−03, A₈ = −4.349E−05,A₁₀ = 1.085E−05, A₁₂ = 1.560E−05, A₁₄ = 2.371E−06, A₁₆ = −6.171E−07Eleventh Surface k = 0.000, A₄ = −3.607E−02, A₆ = 1.638E−03, A₈ =1.660E−04, A₁₀ = 1.373E−05, A₁₂ = 1.855E−06, A₁₄ = 2.700E−07, A₁₆ =−1.163E−07 Twelfth Surface k = 0.000, A₄ = −1.004E−02, A₆ = −8.471E−04,A₈ = 9.471E−05, A₁₀ = 2.438E−06, A₁₂ = −1.674E−06, A₁₄ = −1.484E−07, A₁₆= 2.570E−08 Thirteenth Surface k = 0.000, A₄ = −2.975E−02, A₆ =8.262E−03, A₈ = −1.318E−03, A₁₀ = 1.029E−04, A₁₂ = −1.209E−05, A₁₄ =1.657E−06, A₁₆ = −7.825E−08 Fourteenth Surface k = 0.000, A₄ =−4.975E−02, A₆ = 9.427E−03, A₈ = −1.767E−03, A₁₀ = 2.350E−04, A₁₂ =−2.032E−05, A₁₄ = 9.663E−07, A₁₆ = −1.916E−08 f1 = 4.27 mm f2 = −14.69mm f3 = −35.74 mm f4 = 14.21 mm f5 = −24.58 mm f6 = −100.39 mm f7 =−7.94 mm f23 = −10.26 mm f45 = 32.66 mm f67 = −7.44 mm The values of therespective conditional expressions are as follows: f23/f1 = −2.40 f2/f3= 0.41 f1/f = 0.68 f45/f67 = −4.39 f67/f = −1.18 f4/f = 2.25 f7/f =−1.26 D23/f = 0.19 D34/f = 0.035

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theabove-described conditional expressions. A distance on the optical axisfrom the object-side surface of the first lens L1 to the image plane IM(air conversion length for the filter 10) is 6.92 mm, and downsizing ofthe imaging lens is attained.

FIG. 8 shows the lateral aberration of the imaging lens in NumericalData Example 3, which corresponds to the image height ratio H of theimaging lens. FIG. 9 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively. As shown in FIGS. 8 and 9,according to the imaging lens of Numerical Data Example 3, theaberrations are satisfactorily corrected.

NUMERICAL DATA EXAMPLE 4

Basic data are shown below.

f = 6.58 mm, Fno = 2.7, ω = 31.3° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* (Stop) 2.094 0.709 1.5346 56.1 (=νd1)  2*18.265 0.091  3* 149.869 0.286 1.6355 24.0 (=νd2)  4* 8.321 0.332 (=D23) 5* −44.743 0.474 1.6355 24.0 (=νd3)  6* 631.923 1.233 (=D34)  7*−12.494 0.352 1.5346 56.1 (=νd4)  8* −4.802 0.037  9* 20.999 0.5101.6355 24.0 (=νd5) 10* 8.539 0.294 11* 12.428 0.580 1.5346 56.1 (=νd6)12* 8.742 0.303 13* 22.998 0.523 1.5346 56.1 (=νd7) 14* 3.749 0.200 15 ∞0.200 1.5168 64.2 16 ∞ 0.867 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = −7.892E−04, A₆ = −1.001E−03, A₈ = −1.142E−03,A₁₀ = −1.165E−03, A₁₂ = 6.915E−04, A₁₄ = −2.917E−04, A₁₆ = −2.805E−04Second Surface k = 0.000, A₄ = −7.015E−02, A₆ = 3.117E−02, A₈ =−4.145E−04, A₁₀ = −3.478E−03, A₁₂ = −1.637E−03, A₁₄ = −1.442E−04, A₁₆ =3.801E−04 Third Surface k = 0.000, A₄ = −7.109E−02, A₆ = 6.425E−02, A₈ =−7.337E−03, A₁₀ = −4.771E−03, A₁₂ = −7.589E−04, A₁₄ = 6.636E−04, A₁₆ =3.497E−04 Fourth Surface k = 0.000, A₄ = −1.703E−02, A₆ = 4.106E−02, A₈= −5.205E−03, A₁₀ = −2.994E−03, A₁₂ = 1.503E−03, A₁₄ = 5.453E−04, A₁₆ =1.614E−04 Fifth Surface k = 0.000, A₄ = −4.694E−02, A₆ = 4.353E−03, A₈ =6.064E−04, A₁₀ = 1.506E−03, A₁₂ = 5.856E−04, A₁₄ = 1.643E−04, A₁₆ =1.317E−04 Sixth Surface k = 0.000, A₄ = −3.486E−02, A₆ = 1.909E−03, A₈ =2.397E−03, A₁₀ = 2.932E−04, A₁₂ = −1.362E−05, A₁₄ = 1.947E−04, A₁₆ =−3.844E−05 Seventh Surface k = 0.000, A₄ = −1.544E−02, A₆ = −1.150E−02,A₈ = 7.726E−03, A₁₀ = −3.196E−04, A₁₂ = −3.360E−03, A₁₄ = 1.780E−03, A₁₆= −2.839E−04 Eighth Surface k = 0.000, A₄ = −9.514E−03, A₆ = 8.817E−03,A₈ = −1.361E−03, A₁₀ = −3.560E−03, A₁₂ = 2.668E−03, A₁₄ = −6.826E−04,A₁₆ = 6.002E−05 Ninth Surface k = 0.000, A₄ = −1.786E−02, A₆ =6.056E−03, A₈ = −4.587E−03, A₁₀ = −2.595E−04, A₁₂ = 8.166E−04, A₁₄ =−2.182E−04, A₁₆ = 1.859E−05 Tenth Surface k = 0.000, A₄ = −1.829E−02, A₆= −1.818E−03, A₈ = −9.221E−05, A₁₀ = 9.675E−06, A₁₂ = 1.590E−05, A₁₄ =2.459E−06, A₁₆ = −5.978E−07 Eleventh Surface k = 0.000, A₄ = −3.668E−02,A₆ = 1.601E−03, A₈ = 1.713E−04, A₁₀ = 1.451E−05, A₁₂ = 1.972E−06, A₁₄ =2.845E−07, A₁₆ = −1.146E−07 Twelfth Surface k = 0.000, A₄ = −7.337E−03,A₆ = −8.988E−04, A₈ = 7.228E−05, A₁₀ = 2.606E−07, A₁₂ = −1.753E−06, A₁₄= −1.417E−07, A₁₆ = 2.780E−08 Thirteenth Surface k = 0.000, A₄ =−3.009E−02, A₆ = 8.104E−03, A₈ = −1.311E−03, A₁₀ = 1.031E−04, A₁₂ =−1.212E−05, A₁₄ = 1.653E−06, A₁₆ = −7.866E−08 Fourteenth Surface k =0.000, A₄ = −5.724E−02, A₆ = 9.863E−03, A₈ = −1.759E−03, A₁₀ =2.354E−04, A₁₂ = −2.034E−05, A₁₄ = 9.676E−07, A₁₆ = −1.898E−08 f1 = 4.36mm f2 = −13.87 mm f3 = −65.73 mm f4 = 14.36 mm f5 = −23.01 mm f6 =−58.33 mm f7 = −8.46 mm f23 = −11.41 mm f45 = 36.40 mm f67 = −7.41 mmThe values of the respective conditional expressions are as follows:f23/f1 = −2.62 f2/f3 = 0.21 f1/f = 0.66 f45/f67 = −4.91 f67/f = −1.13f4/f = 2.18 f7/f = −1.29 D23/f = 0.050 D34/f = 0.19

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theabove-described conditional expressions. A distance on the optical axisfrom the object-side surface of the first lens L1 to the image plane IM(air conversion length for the filter 10) is 6.92 mm, and downsizing ofthe imaging lens is attained.

FIG. 11 shows the lateral aberration of the imaging lens in NumericalData Example 4, which corresponds to the image height ratio H of theimaging lens. FIG. 12 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively. As shown in FIGS. 11 and 12,according to the imaging lens of Numerical Data Example 4, theaberrations are also satisfactorily corrected.

NUMERICAL DATA EXAMPLE 5

Basic data are shown below.

f = 5.58 mm, Fno = 2.3, ω = 35.6° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* (Stop) 1.975 0.875 1.5346 56.1 (=νd1)  2*21.088 0.056  3* 64.594 0.300 1.6355 24.0 (=νd2)  4* 7.407 0.425 (=D23) 5* −36.756 0.518 1.6355 24.0 (=νd3)  6* 71.409 0.204 (=D34)  7* −13.8910.481 1.5346 56.1 (=νd4)  8* −4.859 0.138  9* 12.382 0.510 1.6355 24.0(=νd5) 10* 6.912 0.224 11* 27.816 0.601 1.5346 56.1 (=νd6) 12* 8.5080.231 13* 7.135 0.995 1.5346 56.1 (=νd7) 14* 3.387 0.200 15 ∞ 0.2001.5168 64.2 16 ∞ 0.576 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = 4.278E−04, A₆ = −1.403E−03, A₈ = −4.651E−04, A₁₀= −1.011E−03, A₁₂ = 7.232E−04, A₁₄ = −2.463E−04, A₁₆ = −2.287E−04 SecondSurface k = 0.000, A₄ = −6.203E−02, A₆ = 3.291E−02, A₈ = −2.600E−03, A₁₀= −4.935E−03, A₁₂ = −1.725E−03, A₁₄ = 1.706E−04, A₁₆ = 4.540E−04 ThirdSurface k = 0.000, A₄ = −6.750E−02, A₆ = 6.133E−02, A₈ = −8.453E−03, A₁₀= −5.173E−03, A₁₂ = −9.866E−04, A₁₄ = 6.041E−04, A₁₆ = 4.871E−04 FourthSurface k = 0.000, A₄ = −1.742E−02, A₆ = 3.662E−02, A₈ = −2.816E−03, A₁₀= −2.223E−03, A₁₂ = 1.767E−03, A₁₄ = 6.276E−04, A₁₆ = 1.073E−04 FifthSurface k = 0.000, A₄ = −4.626E−02, A₆ = 6.154E−03, A₈ = 4.691E−03, A₁₀= 2.075E−03, A₁₂ = 5.021E−04, A₁₄ = −1.188E−04, A₁₆ = −2.322E−05 SixthSurface k = 0.000, A₄ = −4.184E−02, A₆ = 4.595E−03, A₈ = 6.699E−03, A₁₀= 3.035E−04, A₁₂ = −7.876E−05, A₁₄ = 2.209E−04, A₁₆ = 1.587E−05 SeventhSurface k = 0.000, A₄ = −3.634E−03, A₆ = −3.239E−03, A₈ = 6.387E−03, A₁₀= −2.848E−04, A₁₂ = −3.207E−03, A₁₄ = 1.763E−03, A₁₆ = −2.880E−04 EighthSurface k = 0.000, A₄ = 1.463E−02, A₆ = 7.468E−03, A₈ = −1.421E−03, A₁₀= −3.737E−03, A₁₂ = 2.610E−03, A₁₄ = −6.962E−04, A₁₆ = 6.206E−05 NinthSurface k = 0.000, A₄ = −1.642E−02, A₆ = −2.897E−03, A₈ = −2.927E−03,A₁₀ = −1.681E−04, A₁₂ = 7.413E−04, A₁₄ = −2.229E−04, A₁₆ = 2.081E−05Tenth Surface k = 0.000, A₄ = −2.260E−02, A₆ = −1.110E−03, A₈ =−2.878E−05, A₁₀ = 1.538E−05, A₁₂ = 1.588E−05, A₁₄ = 2.340E−06, A₁₆ =−6.326E−07 Eleventh Surface k = 0.000, A₄ = −2.844E−02, A₆ = 1.759E−03,A₈ = 1.481E−04, A₁₀ = 1.121E−05, A₁₂ = 1.397E−06, A₁₄ = 2.517E−07, A₁₆ =−1.012E−07 Twelfth Surface k = 0.000, A₄ = −6.661E−03, A₆ = −6.014E−04,A₈ = −6.966E−06, A₁₀ = 3.555E−06, A₁₂ = −6.631E−08, A₁₄ = −1.040E−07,A₁₆ = 1.185E−08 Thirteenth Surface k = 0.000, A₄ = −3.493E−02, A₆ =8.293E−03, A₈ = −1.304E−03, A₁₀ = 1.039E−04, A₁₂ = −1.200E−05, A₁₄ =1.657E−06, A₁₆ = −7.841E−08 Fourteenth Surface k = 0.000, A₄ =−4.589E−02, A₆ = 8.687E−03, A₈ = −1.777E−03, A₁₀ = 2.358E−04, A₁₂ =−2.035E−05, A₁₄ = 9.675E−07, A₁₆ = −1.964E−08 f1 = 4.01 mm f2 = −13.19mm f3 = −38.11 mm f4 = 13.72 mm f5 = −25.55 mm f6 = −23.18 mm f7 =−13.29 mm f23 = −9.70 mm f45 = 27.98 mm f67 = −8.13 mm The values of therespective conditional expressions are as follows: f23/f1 = −2.42 f2/f3= 0.35 f1/f = 0.72 f45/f67 = −3.44 f67/f = −1.46 f4/f = 2.46 f7/f =−2.38 D23/f = 0.076 D34/f = 0.037

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theabove-described conditional expressions. A distance on the optical axisfrom the object-side surface of the first lens L1 to the image plane IM(air conversion length for the filter 10) is 6.47 mm, and downsizing ofthe imaging lens is attained.

FIG. 14 shows the lateral aberration of the imaging lens in NumericalData Example 5, which corresponds to the image height ratio H of theimaging lens. FIG. 15 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively. As shown in FIGS. 14 and 15,according to the imaging lens of Numerical Data Example 5, theaberrations are satisfactorily corrected.

NUMERICAL DATA EXAMPLE 6

Basic data are shown below.

f = 7.33 mm, Fno = 4.1, ω = 28.6° Unit: mm Surface Data Surface Number ir d nd νd (Object) ∞ ∞  1* (Stop) −44.829 0.512 1.5346 56.1 (=νd1)  2*−6.136 0.020  3* 5.874 0.214 1.6355 24.0 (=νd2)  4* 4.901 0.293 (=D23) 5* 37.489 0.298 1.6355 24.0 (=νd3)  6* 23.846 0.256 (=D34)  7* 4.5150.669 1.5346 56.1 (=νd4)  8* 41.011 1.923  9* 5.751 0.510 1.6355 24.0(=νd5) 10* 5.095 0.607 11* 8.203 1.236 1.5346 56.1 (=νd6) 12* 5.3780.995 13* 36.754 0.547 1.5346 56.1 (=νd7) 14* 3.631 0.200 15 ∞ 0.2001.5168 64.2 16 ∞ 0.282 (Image ∞ plane) Aspheric Surface Data FirstSurface k = 0.000, A₄ = −1.873E−02, A₆ = −1.109E−02, A₈ = −4.570E−03,A₁₀ = 5.137E−04, A₁₂ = 3.727E−03, A₁₄ = 1.298E−03, A₁₆ = −1.706E−03Second Surface k = 0.000, A₄ = −8.151E−02, A₆ = 2.883E−02, A₈ =−4.255E−03, A₁₀ = −2.454E−03, A₁₂ = −6.209E−04, A₁₄ = 1.801E−04, A₁₆ =1.428E−03 Third Surface k = 0.000, A₄ = −7.671E−02, A₆ = 4.679E−02, A₈ =−1.249E−02, A₁₀ = −3.500E−03, A₁₂ = 9.848E−04, A₁₄ = 8.817E−04, A₁₆ =−2.798E−04 Fourth Surface k = 0.000, A₄ = −4.097E−02, A₆ = 2.460E−02, A₈= −6.438E−03, A₁₀ = −3.589E−03, A₁₂ = 2.222E−04, A₁₄ = −1.119E−04, A₁₆ =2.542E−04 Fifth Surface k = 0.000, A₄ = −6.469E−02, A₆ = 1.564E−02, A₈ =2.618E−03, A₁₀ = −8.544E−04, A₁₂ = −7.128E−04, A₁₄ = −6.448E−05, A₁₆ =2.675E−04 Sixth Surface k = 0.000, A₄ = −4.461E−02, A₆ = 6.629E−04, A₈ =3.310E−03, A₁₀ = 3.010E−04, A₁₂ = −2.561E−04, A₁₄ = 4.397E−05, A₁₆ =−1.793E−07 Seventh Surface k = 0.000, A₄ = 5.814E−03, A₆ = −1.035E−02,A₈ = 4.677E−03, A₁₀ = −1.371E−04, A₁₂ = −2.909E−03, A₁₄ = 1.885E−03, A₁₆= −4.024E−04 Eighth Surface k = 0.000, A₄ = −7.255E−03, A₆ = 1.787E−03,A₈ = −8.541E−04, A₁₀ = −3.526E−03, A₁₂ = 2.579E−03, A₁₄ = −7.317E−04,A₁₆ = 5.890E−05 Ninth Surface k = 0.000, A₄ = −2.248E−02, A₆ =5.783E−04, A₈ = −2.280E−03, A₁₀ = −4.753E−04, A₁₂ = 7.297E−04, A₁₄ =−2.310E−04, A₁₆ = 2.496E−05 Tenth Surface k = 0.000, A₄ = −1.820E−02, A₆= −1.371E−03, A₈ = −3.740E−06, A₁₀ = 2.744E−05, A₁₂ = 1.287E−05, A₁₄ =2.924E−06, A₁₆ = −6.523E−07 Eleventh Surface k = 0.000, A₄ = −2.330E−02,A₆ = 9.404E−04, A₈ = 1.272E−04, A₁₀ = 1.381E−06, A₁₂ = 8.022E−07, A₁₄ =2.478E−07, A₁₆ = −7.384E−08 Twelfth Surface k = 0.000, A₄ = −1.368E−02,A₆ = −5.267E−04, A₈ = 2.149E−05, A₁₀ = 6.717E−06, A₁₂ = −9.850E−07, A₁₄= −1.166E−07, A₁₆ = 1.921E−08 Thirteenth Surface k = 0.000, A₄ =−4.044E−02, A₆ = 8.363E−03, A₈ = −1.297E−03, A₁₀ = 1.043E−04, A₁₂ =−1.196E−05, A₁₄ = 1.664E−06, A₁₆ = −7.712E−08 Fourteenth Surface k =0.000, A₄ = −4.735E−02, A₆ = 8.446E−03, A₈ = −1.735E−03, A₁₀ =2.391E−04, A₁₂ = −2.016E−05, A₁₄ = 9.641E−07, A₁₆ = −2.056E−08 f1 =13.24 mm f2 = −50.93 mm f3 = −103.98 mm f4 = 9.43 mm f5 = −100.75 mm f6= −34.46 mm f7 = −7.58 mm f23 = −34.17 mm f45 = 9.73 mm f67 = −6.28 mmThe values of the respective conditional expressions are as follows:f23/f1 = −2.58 f2/f3 = 0.49 f1/f = 1.81 f45/f67 = −1.55 f67/f = −0.86f4/f = 1.29 f7/f = −1.03 D23/f = 0.040 D34/f = 0.035

Accordingly, the imaging lens of Numerical Data Example 6 satisfies theabove-described conditional expressions. A distance on the optical axisfrom the object-side surface of the first lens L1 to the image plane IM(air conversion length for the filter 10) is 8.69 mm, and downsizing ofthe imaging lens is attained.

FIG. 17 shows the lateral aberration of the imaging lens in NumericalData Example 6, which corresponds to the image height ratio H of theimaging lens. FIG. 18 shows a spherical aberration (mm), astigmatism(mm), and a distortion (%), respectively. As shown in FIGS. 17 and 18,according to the imaging lens of Numerical Data Example 6, theaberrations are satisfactorily corrected.

According to the imaging lens of the embodiment described above, it isachievable to obtain a wide angle of view (2ω) that is 70° or greater.Here, the imaging lenses of Numerical Data Examples 1 to 6 have anglesof view, which are as wide as 57.2° to 72.6°. According to the imaginglens of the embodiment, it is achievable to take an image of a widerrange than the range that can be taken by a conventional imaging lens.

Furthermore, with advancement in digital zooming technology forenlarging any range of an image obtained through an imaging lens byimage processing, a high-pixel count imaging element has been often usedin combination with a high-resolution imaging lens. In case of thosehigh-pixel count imaging lens, a light-receiving area of each pixeldecreases, so that an image taken by the imaging lens tends to be dark.As a method to solve this problem, there is a method of improving alight-receiving sensitivity of an imaging element using an electricalcircuit. However, when a light-receiving sensitivity increases, a noisecomponent that does not contribute to image formation directly is alsoamplified. Therefore, it is necessary to have another circuit to reducenoises. According to the imaging lenses of the embodiment, Fno is assmall as 2.2 to 3.0. According to the imaging lenses of the embodiment,it is achievable to obtain sufficiently bright images without additionalelectrical circuit as described above.

Therefore, when the imaging lens of the embodiment is applied in animaging optical system including a camera for mounting in a portabledevice such as cellular phones, portable information terminals, andsmartphones, a digital still camera, a security camera, an onboardcamera, and network camera, it is possible to attain both highfunctionality of the cameras and downsizing.

The invention is applicable to an imaging lens for mounting in arelatively small-sized camera, including a camera to be equipped in aportable device such as cellular phones and portable informationterminals, a digital still camera, a security camera, a vehicle onboardcamera, and a network camera.

The disclosure of Japanese Patent Application No. 2014-057414, filed onMar. 20, 2014, is incorporated in the application by reference.

While the invention has been explained with reference to the specificembodiments of the invention, the explanation is illustrative and theinvention is limited only by the appended claims.

What is claimed is:
 1. An imaging lens comprising: a first lens group; asecond lens group; and a third lens group, arranged in this order froman object side to an image plane side, wherein said first lens groupincludes a first lens having positive refractive power, a second lenshaving negative refractive power, and a third lens, said second lensgroup includes a fourth lens and a fifth lens, said third lens groupincludes a sixth lens and a seventh lens, said first lens, said secondlens, said third lens, said fourth lens, said fifth lens, said sixthlens, and said seventh lens are arranged on an optical axis with a spacein between respectively, said seventh lens is formed in a shape so thata surface thereof on the object side and a surface thereof on the imageplane side have positive curvature radii, said surface of the seventhlens on the image plane side is formed in an aspheric shape having atleast one inflexion point, and said first lens has an Abbe's number νd1and said third lens has an Abbe's number νd3 so that the followingconditional expressions are satisfied:40<νd1<75,20<νd3<35.
 2. The imaging lens according to claim 1, wherein said secondlens has a focal length f2 and said third lens has a focal length f3 sothat the following conditional expression is satisfied:0.1<f2/f3<0.6.
 3. The imaging lens according to claim 1, wherein saidfourth lens has a focal length f4 so that the following conditionalexpression is satisfied:1.0<f4/f<3.0 where f is a focal length of a whole lens system.
 4. Theimaging lens according to claim 1, wherein said seventh lens has a focallength f7 so that the following conditional expression is satisfied:−4.0<f7/f<−0.8 where f is a focal length of a whole lens system.
 5. Theimaging lens according to claim 1, wherein said second lens is disposedaway from the third lens by a distance D23 on an optical axis thereof sothat the following conditional expression is satisfied:0.03<D23/f<0.2 where f is a focal length of a whole lens system.
 6. Theimaging lens according to claim 1, wherein said third lens is disposedaway from the fourth lens by a distance D34 on an optical axis thereofso that the following conditional expression is satisfied:0.03<D34/f<0.2 where f is a focal length of a whole lens system.
 7. Theimaging lens according to claim 1, wherein said fourth lens has anAbbe's number νd4 and said fifth lens has an Abbe's number νd5 so thatthe following conditional expressions are satisfied:40<νd4<75,20<νd5<35.
 8. The imaging lens according to claim 1, wherein said sixthlens has an Abbe's number νd6 and said seventh lens has an Abbe's numberνd7 so that the following conditional expressions are satisfied:40<νd6<75,40<νd7<75.
 9. An imaging lens comprising: a first lens group; a secondlens group; and a third lens group having negative refractive power,arranged in this order from an object side to an image plane side,wherein said first lens group includes a first lens having positiverefractive power, a second lens, and a third lens having negativerefractive power, said second lens group includes a fourth lens and afifth lens, said third lens group includes a sixth lens and a seventhlens, said first lens, said second lens, said third lens, said fourthlens, said fifth lens, said sixth lens, and said seventh lens arearranged on an optical axis with a space in between respectively, saidsecond lens is formed in a shape so that a surface thereof on the imageplane side has a positive curvature radius, said seventh lens is formedin a shape so that a surface thereof on the image plane side has apositive curvature radius, said surface of the seventh lens on the imageplane side is formed in an aspheric shape having at least one inflexionpoint, and said seventh lens has a focal length f7, said first lens hasan Abbe's number νd1, and said third lens has an Abbe's number νd3 sothat the following conditional expressions are satisfied:−4.0<f7/f<−0.8,40<νd1<75,20<νd3<35, where f is a focal length of a whole lens system.
 10. Theimaging lens according to claim 9, wherein said second lens has a focallength f2 and said third lens has a focal length f3 so that thefollowing conditional expression is satisfied:0.1<f2/f3<0.6.
 11. The imaging lens according to claim 9, wherein saidfourth lens has a focal length f4 so that the following conditionalexpression is satisfied:1.0<f4/f<3.0.
 12. The imaging lens according to claim 9, wherein saidsecond lens is disposed away from the third lens by a distance D23 on anoptical axis thereof so that the following conditional expression issatisfied:0.03<D23/f<0.2.
 13. The imaging lens according to claim 9, wherein saidthird lens is disposed away from the fourth lens by a distance D34 on anoptical axis thereof so that the following conditional expression issatisfied:0.03<D34/f<0.2
 14. The imaging lens according to claim 9, wherein saidfourth lens has an Abbe's number νd4 and said fifth lens has an Abbe'snumber νd5 so that the following conditional expressions are satisfied:40<νd4<75,20<νd5<35.
 15. An imaging lens comprising: a first lens group; a secondlens group; and a third lens group having negative refractive power,arranged in this order from an object side to an image plane side,wherein said first lens group includes a first lens having positiverefractive power, a second lens, and a third lens having negativerefractive power, said second lens group includes a fourth lens and afifth lens, said third lens group includes a sixth lens and a seventhlens, said first lens, said second lens, said third lens, said fourthlens, said fifth lens, said sixth lens, and said seventh lens arearranged on an optical axis with a space in between respectively, saidsixth lens is formed in a shape so that a surface thereof on the objectside has a positive curvature radius, said seventh lens is formed in ashape so that a surface thereof on the image plane side has a positivecurvature radius, said surface of the seventh lens on the image planeside is formed in an aspheric shape having at least one inflexion point,and said second lens is disposed away from the third lens by a distanceD23 on an optical axis thereof, said first lens has an Abbe's numberνd1, and said third lens has an Abbe's number νd3 so that the followingconditional expressions are satisfied:0.03<D3/f<0.2,40<νd1<75,20<νd3<35, where f is a focal length of a whole lens system.
 16. Theimaging lens according to claim 15, wherein said second lens has a focallength f2 and said third lens has a focal length f3 so that thefollowing conditional expression is satisfied:0.1<f2/f3<0.6.
 17. The imaging lens according to claim 15, wherein saidfourth lens has a focal length f4 so that the following conditionalexpression is satisfied:1.0<f4/f<3.0.
 18. The imaging lens according to claim 15, wherein saidseventh lens has a focal length f7 so that the following conditionalexpression is satisfied:−4.0<f7/f<−0.8.
 19. The imaging lens according to claim 15, wherein saidthird lens is disposed away from the fourth lens by a distance D34 on anoptical axis thereof so that the following conditional expression issatisfied:0.03<D34/f<0.2.
 20. The imaging lens according to claim 15, wherein saidfourth lens has an Abbe's number νd4 and said fifth lens has an Abbe'snumber νd5 so that the following conditional expressions are satisfied:40<νd4<75,20<νd5<35.